Pressure-sensitive tapes have been commercially used on a large scale for many years. Such tapes are widely used as maskings during painting, for sealing containers, and many other uses where a removably affixed tape is desirable. Pressure-sensitive tapes are commonly formed using solvent-based adhesives, whereby an adhesive composition is first dissolved in a volatile solvent and the solution is applied to a tape backing, such as a polymeric strip or a creped paper. Upon heating the adhesive-coated tape, the solvent evaporates to leave a pressure-sensitive adhesive on the backing. Although solvent-based adhesives perform satisfactorily, the use of solvents has certain disadvantages. More specifically, the evaporated volatile solvents must be collected and disposed of properly to avoid environmental pollution, and the solvents must not be overheated to avoid igniting the evaporated fumes.
Accordingly, other methods for making pressure-sensitive tapes have been developed that avoid the use of volatile solvents. In particular, hot melt or thermoplastic adhesives can be used as alternatives to solvent-based adhesives. Hot melt adhesives typically are dense, non-solvent based materials comprising a natural or synthetic polymer or polymeric composition and are applied to the backing through extrusion or similar processes. The material is normally solid or semi-solid at room temperature and becomes flowable upon heating to a higher temperature. Upon cooling, the material returns to a solid or semi-solid state.
Some of the early attempts at making masking tapes using hot melt adhesives were less than resounding successes. The early tapes used a solid layer of hot melt adhesive extruded onto the paper backing, which resulted in a slick finish that tended to yield high peel strengths. This was undesirable because it could be difficult to remove the tape from a surface. The early tapes also suffered from the problem of poor adhesion of the adhesive to the paper backing, which could result in the adhesive remaining on the surface to which the tape was applied instead of coming off with the backing when the tape was removed. Some tapes included a primer coating on one side of the tape to improve the bond between the adhesive and the tape, but the tapes still suffered from poor adhesion. These hot melt tapes were readily recognized by the shiny appearance of the adhesive side, and a resistance to these hot melt tapes, or any tape having a shiny appearance, developed in the marketplace.
Another problem with the early hot melt tapes was that the solid layer of hot melt adhesive had less volume compared to an equivalent weight of a solvent-based adhesive. Thus, applying a hot melt adhesive to the paper backing resulted in the tape having an overall thickness that was less than if an equivalent weight of solvent-based adhesive were used. Consequently, the hot melt tape when wound on a roll had a smaller overall diameter than the solvent-based tape, which was also undesirable in the marketplace.
Improvements have been made to hot melt adhesive tapes. In particular, peel strength as well as thickness problems have been improved by foaming the adhesive. Foaming of the adhesive is typically accomplished by adding to the adhesive composition a chemical foaming or blowing agent, which can be broadly classified as either chemical or physical. Chemical foaming or blowing agents form a foam or cell structure by decomposition of the foaming agent when a decomposition temperature is reached. Physical foaming agents, such as chlorinated hydrocarbons, ketones, and alcohols, cause a cell structure to form by evaporation upon heating. In both cases, heating the adhesive-coated tape causes the foaming agents to react and foam the adhesive. Also known are gaseous foaming agents, which are incorporated into a thermoplastic mix at a higher pressure and foam as the pressure is reduced. Thus, foaming or blowing agents cause tiny bubbles to form in the hot melt adhesive, giving the adhesive layer a porous or rough surface finish having improved release characteristics and better aesthetic qualities, such as a desirable dull surface finish.
In some cases moisture present in a release liner, such as a silicone coated paper, has been used as the foaming agent instead of added foaming or blowing agents in the hot melt adhesive. In particular, U.S. Pat. No. 5,605,717 to Simmons et al. ("the Simmons patent") discloses a process for vaporizing the moisture present in a release liner by applying a molten hot melt adhesive to a silicone-coated surface of the release liner to cause the moisture present therein to turn to gas and migrate as small bubbles from the silicone coated paper into the adhesive. However, the Simmons patent is not directed towards masking tapes and thus is not concerned with forming a strong bond between the adhesive and the release liner, as evidenced by providing a silicone coated paper instead of an uncoated paper. In addition, the Simmons patent discloses applying the adhesive in a molten state to the release liner to foam the adhesive. This may be disadvantageous in certain applications, such as when applying a very thin layer of adhesive to a thicker backing, because the backing can act as a heat sink and quickly cool the adhesive to below the boiling point of water such that foaming prematurely ceases.
As described above, chemical and physical foaming or blowing agents often are incorporated into the hot melt adhesive to improve the qualities thereof. However, such agents create additional costs in making the finished tape, as well as adding complexity to the process itself. Moreover, certain high-volatility foaming agents require a relatively high temperature to volatilize, which is undesirable as the high temperature may damage the backing. Simmons has suggested using moisture present in a release paper as a foaming agent but his method may not provide adequate control of the foaming process and may yield only moderate results. It would therefore be desirable to provide a pressure-sensitive masking tape having a foamed hot melt adhesive without the use of chemical or physical foaming or blowing agents. It would also be desirable to provide such a masking tape having a superior bond between the foamed adhesive and paper backing, yet having a desirable peel strength.